Online TA Help
Last updated: 01/04/13
Field Methods in Hydrology
To facilitate your learning of the subject material for this course, this page has been set
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lectures, field trips, or other appropriate topics. Your questions and their answers will be
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likely that several people are wondering about the same thing.
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question archive from previous years' classes. Please take
the time to carefully phrase your question so that it may be properly answered. The more
specific and well-phrased the question is, the more direct and useful the answer will be
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Questions and Answers
HOMEWORK #1 QUESTIONS
Q: What is meant by the centroid of the watershed, and how would we go about finding such a thing?
A: The centroid of something is its "center of mass". That means that if your sketch was made out of stiff paper and you cut it out along the boundary, then the centroid would be the one and only point at which you could balance the paper on the tip of a pin. In a pinch you can guesstimate it visually with that concept in mind.
Q: For part 2 (plotting streamflow and precipitation), how are we supposed to do that? I started out using the highest peak flow of a year and correlated it with the annual precip (for that year). Should we look at each annual peak flow date and use the monthly precipitation data instead? Or is there a better way?
A: You can download daily streamflow data and daily rainfall data and plot them together if the dates line up. Alternately, you can download data from the ALERT network, which has both rainfall data and river *stage* data for many sites. Since discharge is proportional to stage, you can just plot rainfall and stage together.
Q: How do we infer from the Morrison Creek data to analyze our watershed, since it doesn't fall within the boundary of our watershed?
A: Use the annual peak flow data from Morrison Creek to calculate all of the discharges you need (Q50, Q100, etc., where Q=discharge and 50=the 50-yr flow which has a 98% nonexceedence probability). Then use the ratio of the areas of the two watershed to transfer your Morrison Creek answers to Cirby Creek (by way of proportions: Area Cirby/Area Morrison = Q Cirby/Q Morrison). The area of Morrison Creek watershed is given in the USGS information, so you don't even have to look at it on the map. FYI, Morrison Creek is in south Sacramento.
Q: For the last part of the second section, do we calculate Q2, Q10, etc. using the drainage area for Morrison Creek or Linda Creek? I would assume you could do either but since all of section (D) uses information for Morrison Creek I would imagine you want some sort of comparsion between the two sets of values. We were planning to make a weighted average comparison for the difference in watershed areas to compare Q2, Q10, etc. between Morrison and Linda for the nonexceedance probability plot.
A: I recommend computing your Q values using the Morrison Creek data and converting the values to Linda Creek before comparing against the regional regression equations. That way you can apply the regional regression equations directly to Linda Creek.
Q: What is the parameter H in the Wanaanen and Crippen equation?
A: It is an elevation index found by: H= (Max elevation + Min elevation)/2.
Q: What are the units for the time lag (TL)?
A: TL is in units of hours.
Q: Can I outline each soil unit on the map copy or do I have to group them as stated in the directions? If I have to group them, can I group by the units A through D? The reason I ask is because I am having a hard time figuring out how to classify the units by the SCS #.
A: When you look at the soil maps you will see the accompanying book that provides detailed information on each soil type. After you sketch the broad distribution of soil types on your watershed, look up the information that corresponds to those soil types in the book and use that information to determine the corresponding SCS soil type (A-D) as defined in the handout. County soils maps are the best soils info a hydrologist has access to. From that information we must determine broad based infilitration characteristics.
Q: For Part 3 Letter E, I am not completely sure what it is I am supposed to do here. Could you please explain some more?
A: You only have to use the USGS topo maps for this. Those maps were originally surveyed ~40-50 years ago. Subsequent revisions of land cover are made based on aerial photos and are given a different color. For example, the 1980-81 "photorevision" shows all new urbanization in purple. What I am asking you to do is estimate the amount of urban area from the early surveys and the amount of new urban area (purple) in the photorevision. If you are adventurous, you can use the giant 1990 aerial photos of Sacramento to give a 1990 urban area number as well. Think about the significance of urban area to flash flood potential.
HOMEWORK #2 QUESTIONS
Q: We are using the procedure for determining the number of samples required for a field study that you gave us in class, but we are finding sample values of the order of several hundreds after repeating the method twice. The problem is that the student t table doesn't go beyond a degree of freedom of 30. What should we do?
A: The key to the number of samples required is the degree of precision (D) you are imposing on your sampling project. The finer precision you impose, the more samples you need. With respect to the homework, you have 2 choices to solve your problem: a) go to the library and look up a longer "t" table in a statistics textbook (extra credit for anyone who brings me a copy of such a table with as many degrees of freedom as you can find!) or b) relax your D so that you need less than 30 samples to achieve that precision. Obviously, it would be unwise to relax D beyond 1 standard deviation, so be careful if you choose path b!
HOMEWORK #3 QUESTIONS
Q: How should we calculate R? Is it acceptable to just add the width of the channel to the average depth*2 or should we create a plot of depths and do linear interpolation? We cannot assume that R=d because w is not 20* greater than d.
A: If W/D<20, then you need to actually figure out the area and the wetted perimeter. There are many ways to get the area. Mathematically, the best way is to use the trapezoidal rule. Another approach is to make a plot that is correctly scaled 1:1 for vertical:horizontal. Then you can cut that out and weigh it. If you also plot and weigh a square area of known scaled dimensions, then you can use weight proportionality to get the cross-section area. Third, you can go on-line and fine some of the available tools that take in cross-sections and give you back metrics like the area. For the perimeter, analogous approaches exist. You could make a scaled line plot and a reference line of known scaled length. Then you could work out how many times it takes to move the reference line along the perimeter to get the perimeter for that water stage. This works best if you make a reference string that can conform to the XS shape.
Q: Where does the time component of V/Q come from in Manning's equation?
A: If you look in your lecture notes, you'll find that the derivation of Chezy's equation has a "g" variable in it. That variable has units of L/T^2. That number gets incorporated into the smoothness coefficient, so it is not explicit in the final form. Similarly, for Manning's equation, time is incorporated into the roughness coefficient.
Q: For the slope-area calculations in C2 to determine discharge, should we be using the slope from specifically where our cross-section was measured? Or the slope calculated from the larger longitudinal reach we determined from the class data? It seems like we should be using the slope just at the cross section, but I don't know which data corresponds to where our cross section was measured. If that was the case, do you really only need two points?
A: Manning's and Chezy's equations assume steady, uniform flow. Since that doesn't strictly apply, then the idea is to average over a long enough reach. Of course, your data is only from one cross-section. I suggest that you test different slope values calculated using the field data to see how they perform against the known discharge. never rely on just 2 points to calculate a slope, since there are local bedforms. You have to get the slope form the regression line of multiple points.
Q: What are the units of u*?
A: u* has units of velocity [L/T]. Therefore u/u* is dimensionless. In the log-profile equation, R/ks both have dimensions of [L], so that term is dimensionless. Thus, both sides of the equation are dimensionless, making the equation dimensionally homogeneous.
HOMEWORK #4 QUESTIONS
Q: What is the scale for the map of Schubert Watershed included with the Experiment?
A: The scale is 1 inch equals 650 feet.
Q: What is the time frame for the flow units in the historical Schubert Watershed data?
A: The flow meaurements for the October 97 Schubert watershed data are volumes PER DAY (i.e. m3 per day and liters per day).
FINAL EXAM QUESTIONS
Q: Looking at the practice exam, the first question is regarding a means of distinguishing silt from clay in the field. I think that must have been covered in the Friday lab I missed or during the Drilling demonstraion. At the demonstration, they mentioned that if you roll the soil into a ball and let it dry, silt will crumble and clay will be very hard. I have also been told that silt feels slippery and clay kind of sticky. Did you have another method?
A:We did discuss this in class, but you should check out chapter 6 in the textbook. That practice question aims to emphasize the fact that you are responsible for the material in the course readings. There is a whole "box" devoted to this question.
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